Re: [Xen-devel] [RFC PATCH 2/2] x86/mm: Add mem access rights to NPT

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

Ping

Suravee / Brian / Boris any ideas on this topic are appreciated.

Regards,
Alex

On 27.09.2018 13:37, George Dunlap wrote:
> On 09/26/2018 06:22 PM, Andrew Cooper wrote:
>> On 26/09/18 17:47, George Dunlap wrote:
>>> From: Isaila Alexandru <aisaila@xxxxxxxxxxxxxxx>
>>>
>>> This patch adds access control for NPT mode.
>>>
>>> There aren’t enough extra bits to store the access rights in the NPT p2m
>>> table, so we add a radix tree to store extra information.
>>
>> I'm sorry to re-open this argument, but why?
>>
>> ISTR there being some argument based on pagetable sharing with the
>> IOMMU, but that doesn't work at the moment and can't reasonably be made
>> to work.  For one, attempting to use pt sharing will break as soon as
>> you try and DMA to a mapped grant.
>>
>> I'm disinclined to let a broken vestigial feature get in the way of real
>> improvements.
>>
>> Beyond that, an NPT PTE has basically the same number of software
>> available bits as an EPT PTE.
>>
>> Am I missing anything?
> 
> Wow -- looks like IOMMU/p2m sharing has been disabled unconditionally
> since 2014.  If nobody has complained since then, that seems like a good
> enough reason to me to rip it out.
> 
> Suravee / Brian / Boris -- any opinions?
> 
> The main reason to go with the 'extra bits' solution rather than the
> 'rip out iommu/p2m sharing' solution is because people have been
> prognosticating for years that we would be running out of bits and need
> more at some point in the future.  I thought Paul, for instance, might
> have a use for the extra bits.  But I'm happy to wait until such time as
> we need it and then fish this patch out of the mail archives.
> 
>   -George
> 
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